Capacitive electromechanical transducer

ABSTRACT

The present invention relates to an electromechanical transducer capable of arbitrarily varying the amount of deflection of a vibrating membrane for every element. 
     The electromechanical transducer includes a plurality of elements including at least one cell that includes a first electrode and a second electrode opposed to the first electrode with a gap sandwiched therebetween and a direct-current voltage applying unit configured to be provided for each element and to separately apply a direct-current voltage to the first electrodes in each element. The first electrodes and the second electrodes are electrically separated for every element.

TECHNICAL FIELD

The present invention relates to a capacitive electromechanicaltransducer, such as a capacitive micromachined ultrasonic transducer.

BACKGROUND ART

Capacitive micromachined ultrasonic transducers (CMUTs) are proposed aselectromechanical transducers that perform at least either oftransmission and reception of ultrasonic waves (for example, refer toPCT Japanese Translation Patent Publication No. 2003-527947). The CMUTsare manufactured by using a Micro Electro Mechanical Systems (MEMS)process to which a semiconductor process is applied.

FIG. 7 is a schematic cross-sectional view of a typical CMUT. Referringto FIG. 7, a set of a first electrode 102 and a second electrode 105 iscalled a cell. The first electrode 102 is opposed to the secondelectrode 105 with a vibrating membrane 101 and a gap 104 sandwichedtherebetween. The vibrating membrane 101 is supported by a supporter 103formed on a substrate 106. All the first electrodes 102 are electricallyconnected to each other in the CMUT. A certain level of direct-current(DC) voltage is uniformly applied to the first electrode 102 so that adesired potential difference is generated between the first electrode102 and the second electrode 105. The second electrodes 105 areelectrically separated for every element (a collection of cells). Analternating-current (AC) drive voltage is applied to the secondelectrode 105 to produce an AC electrostatic attraction between thefirst and second electrodes, which vibrates the vibrating membrane 101at a certain frequency to transmit ultrasonic waves. In addition, thevibrating membrane 101 vibrates in response to the ultrasonic waves thatare received to generate a minute electric current caused byelectrostatic induction at the second electrode 105. The value of theelectric current can be measured to acquire a reception signal for everyelement.

The characteristics in the transmission and reception of ultrasonicwaves are determined by the amount of deflection of the vibratingmembrane 101 when the DC voltage is applied to the first electrodes 102.The pressure in the gap 104 of each cell is normally lower than theatmospheric pressure, and the vibrating membrane 101 is deflected towardthe substrate 106 due to the difference between the atmospheric pressureand the pressure in the gap 104. The amount of deflection of thevibrating membrane 101 is determined by mechanical characteristics basedon parameters including the size, shape, thickness, and material of thevibrating membrane 101. When the CMUT is operated, in order to increasethe efficiency of the transmission and reception of ultrasonic waves, acertain potential difference is applied between the two electrodes tocause an electrostatic attraction between the electrodes. The vibratingmembrane 101 is further deflected toward the substrate 106 due to thiselectrostatic attraction. In the transmission of ultrasonic waves, theefficiency of the transmission and reception is increased with thedecreasing distance between the electrodes because the electrostaticattraction is in inverse proportion to the square of the distance. Incontrast, in the reception of ultrasonic waves, the efficiency of thetransmission and reception is also increased with the decreasingdistance between the electrodes because the magnitude of the detectedminute electric current is in inverse proportion to the distance betweenthe electrodes and is in proportion to the potential difference betweenthe electrodes.

Citation List

Patent Literature

PTL 1: PCT Japanese Translation Patent Publication No. 2003-527947

SUMMARY OF INVENTION

However, increasing the potential difference between the two electrodescauses a force caused by the electrostatic force and the difference inpressure between the electrodes to exceed the restoration force of themechanical characteristics of the vibrating membrane. As a result, thevibrating membrane is in contact with the electrode on the substrate tomake a collapse state and, thus, the characteristics of the CMUT aregreatly varied. Accordingly, when the CMUT is normally operated (is notdriven in the collapse state), the potential difference between theelectrodes is set so that a high efficiency of the transmission andreception is achieved and the vibrating membrane has an amount ofdeflection that does not cause the collapse state.

All the first electrodes are electrically connected to each other in theCMUT in related art. Accordingly, a uniform voltage is applied to thefirst electrodes during the operation of the CMUT. Since theabove-mentioned parameters of the vibrating membranes are varied due tovarious factors in manufacture in the CMUT, a variation in the amount ofdeflection of the vibrating membrane is caused even with no potentialdifference between the electrodes. In addition, the amount of deflectionof the vibrating membrane during the operation of the CMUT is alsovaried.

As a result, the efficiency of the transmission and reception is shiftedfrom an expected value and/or the collapse state is sometimes caused insome of the cells to greatly vary the transmission and receptioncharacteristics for every element. In order to resolve the aboveproblems, the present invention provides a capacitive electromechanicaltransducer capable of arbitrarily varying the amount of deflection ofthe vibrating membrane for every element.

According to an embodiment of the present invention, anelectromechanical transducer includes a plurality of elements includingat least one cell that includes a first electrode and a second electrodeopposed to the first electrode with a gap sandwiched therebetween and adirect-current voltage applying unit configured to be provided for eachelement and to separately apply a direct-current voltage to the firstelectrodes in each element. The first electrodes and the secondelectrodes are electrically separated for every element.

According to the present invention, it is possible to arbitrarily adjustthe amount of deflection of the vibrating membrane in theelectromechanical transducer for every element.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of an electromechanicaltransducer according to a first embodiment of the present invention.

FIG. 2 illustrates an example of the configuration of theelectromechanical transducer according to the first embodiment of thepresent invention.

FIG. 3 is a cross-sectional view of an electromechanical transduceraccording to a second embodiment of the present invention.

FIG. 4 illustrates an example of the configuration of anelectromechanical transducer according to a third embodiment of thepresent invention.

FIG. 5 illustrates an example of the configuration of anelectromechanical transducer according to a fourth embodiment of thepresent invention.

FIG. 6A illustrates an example of the configuration of anelectromechanical transducer according to a fifth embodiment of thepresent invention.

FIG. 6B illustrates another example of the configuration of theelectromechanical transducer according to the fifth embodiment of thepresent invention.

FIG. 7 is a schematic cross-sectional view of a capacitiveelectromechanical transducer in related art.

DESCRIPTION OF EMBODIMENTS

Capacitive electromechanical transducers according to embodiments of thepresent invention will herein be described in detail with reference tothe attached drawings.

First Embodiment

FIG. 1 is a schematic cross-sectional view of an electromechanicaltransducer according to a first embodiment of the present invention. Thefirst electrode is called an upper electrode and the second electrode iscalled a lower electrode in the embodiments of the present invention. Avibrating membrane 101 on which an upper electrode 102 is formed issupported by a supporter 103 formed on a substrate 106. The vibratingmembrane 101 vibrates along with the upper electrode 102. A lowerelectrode 105 is formed on the substrate 106 at a position that isopposed to the upper electrode 102 on the vibrating membrane 101 with agap 104 sandwiched therebetween. A composition including the upperelectrode and the lower electrode, which are opposed to each other withthe gap 104 sandwiched therebetween, is called a cell 107 in theembodiments of the present invention. An element 108 includes at leastone cell 107. Specifically, the element 108 is a composition includingone cell 107 or including multiple (at least two) cells that areelectrically connected (connected in parallel) to each other. Althoughtwo cells compose one element in FIG. 1, the present invention is notlimited to this configuration. Multiple cells may be connected to eachother in a two-dimensional array pattern. Multiple (two or more)elements are formed in the electromechanical transducer.

The upper electrode used in the embodiments of the present invention maybe made of at least one kind of metals including aluminum (Al), chromium(Cr), titanium (Ti), gold (Au), platinum (Pt), copper (Cu), silver (Ag),tungsten (W), molybdenum (Mo), tantalum (Ta), and Nickel (Ni) and alloysincluding AlSi, AlCu, AlTi, MoW, and AlCr. The upper electrode may beprovided on at least one of on the upper face of the vibrating membrane,on the rear face of the vibrating membrane, and inside the vibratingmembrane or, when the vibrating membrane is made of a conductivematerial or a semiconductor material, the vibrating membrane itself mayserve as the upper electrode. The lower electrode used in theembodiments of the present invention may be made of the same metal oralloy as that of the upper electrode. When the substrate is made of asemiconductor material, such as silicon, the substrate may serve as thelower electrode.

FIG. 2 illustrates an example of the configuration of theelectromechanical transducer according to the first embodiment of thepresent invention. The electromechanical transducer according to thefirst embodiment is characterized in that not only the lower electrodes105 but also the upper electrodes 102 are electrically separated forevery element. In each element, the multiple upper electrodes 102 areelectrically connected to each other and the multiple lower electrodes105 are electrically connected to each other. Although the upperelectrodes in each element are separately formed for every cell and areelectrically connected to each other via wiring lines (not shown) formedon the vibrating membrane in the first embodiment, one upper electrodemay be formed for every element. The lower electrodes may also beseparately formed for every cell, as in the first embodiment, or onelower electrode may be formed for every element.

A DC voltage applying unit 201 is connected to the upper electrodes 102in each element. The DC voltage applying unit 201 applies a desiredlevel of voltage to the upper electrodes for every element to cause apotential difference from the voltage of the lower electrodes betweenthe upper and lower electrodes. The amount of deflection of thevibrating membranes 101 is determined by this potential difference. Adrive detecting unit 202 is connected to the lower electrodes 105 ineach element. The drive detecting unit 202 includes an AC voltagegenerating unit 203, a current detecting unit 205, and a protectionswitch 204.

When the electromechanical transducer includes N-number elements, the DCvoltage applying units 201 of the N-number are included in theconfiguration of the first embodiment. The drive detecting units 202 ofthe same number as that of the elements are also included in theconfiguration of the first embodiment.

An operation of each drive detecting unit 202 in transmission ofultrasonic waves and an operation thereof in reception of ultrasonicwaves will now be described. In the transmission of ultrasonic waves, anAC voltage is applied by the AC voltage generating unit 203 connected tothe lower electrodes 105. The application of the AC voltage causes an ACpotential difference between the upper electrodes 102 and the lowerelectrodes 105 to produce an AC electrostatic attraction on thevibrating membrane 101. The electrostatic attraction causes thevibrating membrane 101 to vibrate to transmit the ultrasonic waves.Since the protection switch 204 connected to the lower electrodes 105 isturned off in the transmission of the ultrasonic waves, an input part ofthe current detecting unit 205 is protected from the voltage generatedby the AC voltage generating unit 203.

In the reception of ultrasonic waves, the AC voltage generating unit 203is in a high-impedance state and has no effect on the electric potentialof the lower electrodes 105. The protection switch 204 is turned on toconnect the lower electrodes 105 to an input part of the AC voltagegenerating unit 203. The ultrasonic waves that are externally appliedcause the vibrating membrane 101 to vibrate to vary the electrostaticcapacitance between the upper and lower electrodes. Since the upperelectrodes are fixed to a certain electric potential, the inductioncharge occurring at the lower electrodes 105 causes a minute electriccurrent to pass through the wiring line for the lower electrodes 105. Avariation in the minute electric current can be detected by the currentdetecting unit 205 to detect the magnitude of the ultrasonic wavecausing the variation in the capacitance. The electric potential of thelower electrodes is fixed to a certain value by the drive detecting unit202 except when ultrasonic waves are transmitted.

Since not only the lower electrodes but also the upper electrodes areelectrically separated for every element and the DC voltage applyingunit is connected to the upper electrodes for every element in theconfiguration of the first embodiment, it is possible to separatelyapply the DC voltage to the upper electrodes in each element.Accordingly, different electrostatic attractions can be applied todifferent elements to adjust the amount of deflection of the vibratingmembranes. Consequently, it is possible to reduce the variation in thetransmission and reception characteristics of the ultrasonic waves.

Although the DC voltage applying unit 201 is connected to the upperelectrodes 102 and the current detecting unit 205 is connected to thelower electrodes 105 in the first embodiment, the DC voltage applyingunit may be connected to the lower electrodes and the current detectingunit may be connected to the upper electrodes. In addition, theconfiguration of the drive detecting unit is not limited to the onedescribed in this description, another configuration, only theconfiguration used for the transmission, or only the configuration usedfor the reception may be used.

Second Embodiment

A second embodiment of the present invention will now be described withreference to FIG. 3. The second embodiment concerns the configuration ofwiring lines from the upper electrodes and the lower electrodes. Theremaining configuration in the second embodiment is the same as in thefirst embodiment.

FIG. 3 is a cross-sectional view of an electromechanical transduceraccording to the second embodiment of the present invention. A throughwiring line substrate 303 includes two kinds of wiring lines passingthrough the substrate: a lower-electrode through wiring line 301(second-electrode through wiring line) and an upper-electrode throughwiring line 302 (first-electrode through wiring line). All the lowerelectrodes 105 in each element are connected to one lower-electrodethrough wiring line 301. The lower-electrode through wiring line 301extends from the face of the through wiring line substrate 303 towardthe lower electrodes to the face thereof toward a printed circuit boardto be connected to the corresponding bump electrode 304. The upperelectrodes 102 in each element are also connected to one upper-electrodethrough wiring line 302, which is connected to the corresponding bumpelectrode 304.

A bump 305 is formed on each of the bump electrodes 304 formed on theface of the through wiring line substrate 303 toward the printed circuitboard. The lower-electrode through wiring line 301 and theupper-electrode through wiring line 302 are connected to wiring lines ona printed circuit board 306 via the bumps 305. An electrical signal fromeach of the lower electrodes 105 is supplied to the drive detecting unit202 through the wiring line on the printed circuit board 306electrically connected to the lower-electrode through wiring line 301.An electrical signal from each of the upper electrode 102 is supplied tothe DC voltage applying unit 201 through the wiring line on the printedcircuit board 306 electrically connected to the upper-electrode throughwiring line 302.

The configuration in the second embodiment is characterized by thepresence of the lower-electrode through wiring lines 301 and theupper-electrode through wiring lines 302 of the same number as that ofelements. The presence of the lower-electrode through wiring lines 301and the upper-electrode through wiring lines 302 of the same number asthat of elements allows the wiring lines for the upper electrodes to beled to the rear face of the through wiring line substrate with thewiring lines for the upper electrodes 102 separated for every elementeven when the multiple elements are provided. Accordingly, it ispossible to connect the wiring lines for the upper electrodes to themultiple DC voltage applying units 201 with little reduction in the areaof the elements used for the transmission and reception of ultrasonicwaves (with little reduction in the efficiency of the transmission andreception).

Third Embodiment

A third embodiment of the present invention will now be described withreference to FIG. 4. The third embodiment is characterized in that acontrol-signal generating unit is provided to instruct a DC voltage tobe applied to the DC voltage applying unit 201. The remainingconfiguration in the third embodiment is the same as in either of thefirst and second embodiments.

FIG. 4 illustrates an example of the configuration of anelectromechanical transducer according to the third embodiment of thepresent invention. A vibrating-membrane state detecting unit 401 detectsthe amount of deflection of the vibrating membranes 101 (this isequivalent to detection of the distance between the upper electrodes andthe corresponding lower electrodes). Since the magnitude of a currentdetected at each lower electrode is in reverse proportion to the squareof the distance between the upper electrode and the lower electrode(hereinafter simply referred to as an electrode distance) and is inproportion to the potential difference between the electrodes, thecurrent detecting unit that is connected to the lower electrodes servesas the vibrating-membrane state detecting unit in the third embodiment.

The use of the vibrating-membrane state detecting unit 401 of the thirdembodiment allows a difference in the amount of deflection of thevibrating membranes between the elements to be detected by, for example,externally transmitting ultrasonic waves of a single frequency anddetecting the current output from the lower electrodes for everyelement. Alternatively, an AC voltage may be superimposed on the DCvoltage to be applied to the upper electrodes to detect the currentgenerated by the superimposed AC voltage (described below as a fourthembodiment). A unit other than the unit for detecting the current may beused as the vibrating-membrane state detecting unit to, for example,directly measure the amount of deflection of the vibrating membranes.Specifically, a method of detecting the deflection of the vibratingmembrane by using a piezoresistive effect or a method of opticallydetecting the amount of deflection may be used.

A signal detected by each vibrating-membrane state detecting unit 401 issupplied to the corresponding control-signal generating unit 402. Thecontrol-signal generating unit 402 supplies a signal to instruct a DCvoltage to be applied to the DC voltage applying unit 201 on the basisof the detected signal so that the vibrating membrane 101 in the CMUThas a desired amount of deflection. The DC voltage applying unit 201generates a DC voltage on the basis of the signal instructed by thecontrol-signal generating unit 402 and applies the generated DC voltageto the upper electrodes 102. The DC voltage applying unit 201 performingthe above operation can be easily formed by the use of, for example, avoltage-control-signal transmitting circuit and a capacitor.

According to the third embodiment, it is possible to detect the amountof deflection of the vibrating membranes 101 for every element. Inaddition, since the DC voltage can be applied to the upper electrodes ineach element so as to reduce a variation in the amount of deflectionbetween the elements, the difference in the amount of deflection betweenthe vibrating membranes can be further reduced. Furthermore, even if theparameters affecting the vibrating membranes 101 are varied clue to thevariation with time and/or the change in the environment, it is possibleto adjust the state of the vibrating membranes 101 for every element.

Fourth Embodiment

A fourth embodiment of the present invention will now be described withreference to FIG. 5. The fourth embodiment is characterized in that anAC-voltage superimposing unit 403 is provided to superimpose an ACvoltage on the DC voltage to be applied to the upper electrodes. Theremaining configuration in the fourth embodiment is the same as in thethird embodiment. FIG. 5 illustrates an example of the configuration ofan electromechanical transducer according to the fourth embodiment ofthe present invention.

One AC-voltage superimposing unit 403 is connected to the upperelectrodes 102 in each element. The AC-voltage superimposing unit 403superimposes an AC voltage on the DC voltage to be applied to the upperelectrodes 102 by the DC voltage applying unit 201 only during a periodin which the amount of deflection of the vibrating membranes is detected(the variation in the amount of deflection is measured). The AC voltagesuperimposed on the DC voltage to be applied to the upper electrodes 102induces an electric charge on each of the lower electrodes 105 even ifthe vibrating membrane 101 does not vibrate to cause a current from thelower electrode 105. The current has a value of the magnitudecorresponding to the electrode distance between the upper electrode 102and the lower electrode 105 if the AC voltage to be superimposed has aconstant level. Accordingly, the detection of the current by the currentdetecting unit 205 allows the amount of deflection of the vibratingmembranes 101 to be detected as the electrode distance.

The AC voltage to be superimposed may have a frequency that is not equalto the frequency at which the vibrating membrane 101 vibrates. Thisallows only the electrode distance to be detected without vibrating thevibrating membrane 101 with the AC voltage that is superimposed.

A signal switching unit 404 is connected to an output part of eachcurrent detecting unit 205. The signal switching unit 404 supplies anoutput signal to the control-signal generating unit 402 during a periodin which the variation in the amount of deflection is measured. Incontrast, the signal switching unit 404 supplies an output signal to,for example, an external image processing apparatus as the output fromthe sensor during a period in which the variation in the amount ofdeflection is not measured (the ultrasonic wave is measured on the basisof the vibration of the vibrating membrane). The provision of the signalswitching unit 404 allows the current detecting unit 205 to be used bothin the measurement of the variation in the amount of deflection and inthe measurement of ultrasonic waves.

The control-signal generating unit 402 supplies a signal instructing theDC voltage to the DC voltage applying unit 201 on the basis of thereceived signal so that the vibrating membranes 101 have a desiredamount of deflection.

With the configuration of the fourth embodiment, the superimposition ofthe AC voltage and the detection of the current generated on the lowerelectrodes by the current detecting unit allow the amount of deflectionof the vibrating membranes to be detected for every element. Inaddition, setting the frequency of the AC voltage to be superimposed toa value that is not equal to the value of the frequency at which thevibrating membrane vibrates allows the amount of deflection of thevibrating membranes (the electrode distance between the upper and lowerelectrodes) to be detected without vibrating the vibrating membrane.Accordingly, mechanical vibration characteristics of the vibratingmembrane can be removed to achieve the measurement with a higheraccuracy. Since the DC voltage can be applied to the upper electrodesfor every element so as to reduce the variation in the amount ofdeflection between the elements, it is possible to reduce the differencein the amounts of deflection between the vibrating membranes. Inaddition, it is possible to adjust the amount of deflection of thevibrating membranes 101 for every element even if the parametersaffecting the vibrating membranes 101 are varied due to the variationwith time and/or the change in the environment. Furthermore, since thecurrent detecting unit 205 can be used both in the measurement of thevariation in the amount of deflection and in the measurement ofultrasonic waves, it is possible to realize the electromechanicaltransducer with a simple configuration.

Fifth Embodiment

A fifth embodiment of the present invention will now be described withreference to FIGS. 6A and 6B. The fifth embodiment is characterized inthat the current detecting unit 205 switches circuit parameters betweenin the measurement of the variation in the amount of deflection and inthe measurement of ultrasonic waves. The remaining configuration in thefifth embodiment is the same as in the fourth embodiment.

A transimpedance circuit, which is a current-voltage conversion circuitthat converts a change in a minute electric current into a voltage, isused to describe the fifth embodiment. FIGS. 6A and 6B illustrateexamples of the configuration of the transimpedance circuit, which isthe current detecting unit 205 according to the fifth embodiment.Referring to FIGS. 6A and 6B, reference numeral 601 denotes anoperational amplifier (op-amp), reference numerals 602, 604, and 606denote resistors, reference numerals 603, 605, and 607 denotecapacitors, and reference numeral 608 denotes a circuit-elementswitching unit.

In the examples in FIGS. 6A and 6B, the op-amp 601 is connected topositive and negative power supplies. An operation in the measurement ofultrasonic waves will now be described with reference to FIG. 6A. Aninverting input terminal −IN of the op-amp 601 is connected to the upperelectrodes 102 via the protection switch 204. An output terminal OUT ofthe op-amp 601 is connected to the inverting input terminal −IN via theresistor 602 and the capacitor 603, which are connected in parallel toeach other, and the circuit-element switching unit 608 to feed back anoutput signal. A non-inverting input terminal +IN of the op-amp 601 isconnected to a ground terminal via the resistor 604 and the capacitor605, which are connected in parallel to each other. The voltage of theground terminal is equal to an intermediate value between the voltagevalue of the positive power supply and the voltage value of the negativepower supply. The resistor 602 has the same resistance value as that ofthe resistor 604 and the capacitor 603 has the same capacitance value asthat of the capacitor 605. The resistance values of the resistors 602and 604 and the capacitance values of the capacitors 603 and 605 areparameters matched with the specifications in the measurement ofultrasonic waves.

An operation in the measurement of the variation in the amount ofdeflection will now be described with reference to FIG. 6B. In themeasurement of the variation in the amount of deflection, thecircuit-element switching unit 608 is switched and the feedback of anoutput signal is performed by the resistor 606 and the capacitor 607,which are connected in parallel to each other. The resistance value ofthe resistor 606 and the capacitance value of the capacitor 607 areparameters matched with the specifications in the measurement of thevariation in the amount of deflection.

According to the fifth embodiment, it is possible to perform the currentdetection in accordance with the specifications including the frequencyand the magnitude of the current used in the measurement of thevariation in the amount of deflection and the specifications includingthe frequencies of the ultrasonic waves and the magnitude of the currentused in the measurement of ultrasonic waves. Although thecircuit-element switching unit 608 is used only in the feedback part ofthe op-amp 601 in the examples in FIGS. 6A and 6B, the sameconfiguration may be used between the non-inverting input terminal +INof the op-amp and the ground terminal to switch element constants inaccordance with the switch between in the measurement of the variationin the amount of deflection and in the measurement of the ultrasonicwaves.

Although the vibrating membrane 101 is described to be operated in aconventional mode in which the gap between the vibrating membrane 101and the lower electrode 105 constantly exists during the transmissionand reception operation of the electromechanical transducer in the aboveembodiments, the present invention is not limited to the aboveoperation. The present invention is applicable to the operation inanother state, such as a collapse mode in which part of the gap betweenthe vibrating membrane 101 and the lower electrode 105 is eliminated.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-146936, filed Jun. 19, 2009, which is hereby incorporated byreference herein in its entirety.

Reference Signs List

101 Vibrating membrane

102 Upper electrode (first electrode)

103 Supporter

104 Gap

105 Lower electrode (second electrode)

106 Substrate

201 DC voltage applying unit

202 Drive detecting unit

203 AC voltage generating unit

204 Protection switch

205 Current detecting unit

301 Lower-electrode through wiring line

302 Upper-electrode through wiring line

303 Through wiring line substrate

401 Vibrating-membrane state detecting unit

402 Control-signal generating unit

403 AC-voltage superimposing unit

404 Signal switching unit

608 Circuit-element switching unit

1. An electromechanical transducer comprising: a plurality of elementsincluding at least one cell that includes a first electrode and a secondelectrode opposed to the first electrode with a gap sandwichedtherebetween, the first electrodes and the second electrodes beingelectrically separated for every element; and a direct-current voltageapplying unit configured to be provided for each element and toseparately apply a direct-current voltage to the first electrodes ineach element.
 2. The electromechanical transducer according to claim 1,wherein the number of the first electrodes is the same as that of thesecond electrodes in each element.
 3. The electromechanical transduceraccording to claim 1, further comprising: a substrate configured to haveone first-electrode through wiring line and one second-electrode throughwiring line formed therein for every element.
 4. The electromechanicaltransducer according to claim 3, further comprising: a current detectingunit configured to be provided for each element, to be connected to thesecond electrodes in each element, and to detect a current output fromthe second electrodes; and a control-signal generating unit configuredto be provided for each element and to instruct a direct-current voltageto be applied to the direct-current voltage applying unit on the basisof the detected current.
 5. The electromechanical transducer accordingto claim 4, further comprising: an alternating-current voltagesuperimposing unit configured to be provided for each element, to beconnected to the first electrodes in each element, and to superimpose analternating-current voltage on the direct-current voltage, wherein acurrent caused by the alternating-current voltage is detected by thecurrent detecting unit and the direct-current voltage is instructed tothe control-signal generating unit on the basis of the current outputfrom the alternating-current voltage.
 6. The electromechanicaltransducer according to claim 5, further comprising: a signal switchingunit configured to be provided for each element and to switch an outputsignal from the current detecting unit so as to be connected to thecontrol-signal generating unit in detection of a current output from thealternating-current voltage and so as not to be connected to thecontrol-signal generating unit in detection of a current output from thevibration of a vibrating membrane on which the first electrode or thesecond electrode is formed.
 7. The electromechanical transduceraccording to claim 2, further comprising: a substrate configured to haveone first-electrode through wiring line and one second-electrode throughwiring line formed therein for every element.
 8. The electromechanicaltransducer according to claim 7, further comprising: a current detectingunit configured to be provided for each element, to be connected to thesecond electrodes in each element, and to detect a current output fromthe second electrodes; and a control-signal generating unit configuredto be provided for each element and to instruct a direct-current voltageto be applied to the direct-current voltage applying unit on the basisof the detected current.
 9. The electromechanical transducer accordingto claim 8, further comprising: an alternating-current voltagesuperimposing unit configured to be provided for each element, to beconnected to the first electrodes in each element, and to superimpose analternating-current voltage on the direct-current voltage, wherein acurrent caused by the alternating-current voltage is detected by thecurrent detecting unit and the direct-current voltage is instructed tothe control-signal generating unit on the basis of the current outputfrom the alternating-current voltage.
 10. The electromechanicaltransducer according to claim 9, further comprising: a signal switchingunit configured to be provided for each element and to switch an outputsignal from the current detecting unit so as to be connected to thecontrol-signal generating unit in detection of a current output from thealternating-current voltage and so as not to be connected to thecontrol-signal generating unit in detection of a current output from thevibration of a vibrating membrane on which the first electrode or thesecond electrode is formed.